The present invention relates to a brake pressure control device for a road vehicle with hydraulic multi-circuit brake system.
Such a brake pressure control device in a brake system for generating brake pressures to be coupled into the wheel brakes, a main cylinder is provided which can be actuated by a pedal controlled pneumatic braking force amplifier to the brake pressure control device is also provided in the system and has a switch-over device which responds to the actuation behavior of the driver and causes the braking force amplifier to be automatically switched over from a value of the amplification factor meeting the requirement for judicious braking to a higher value of the amplification factor meeting the requirement for full braking at least in the case where the speed with which the driver actuates the brake pedal in the introductory phase of a braking action exceeds a threshold value .phi..sub.s. Such a brake pressure control device is described in DE 42 08 496 C1.
In such a brake pressure control device, the vacuum braking force amplifier is provided with a magnetic valve arrangement which can be driven, by triggering via an electrical output signal of the electronic control unit, from a basic position, in which a control passage, via a pressure balance can take place between the vacuum chamber and the actuation chamber of the braking force amplifier, of the control part of the braking force amplifier is in communicative connection with the vacuum chamber but the actuation chamber is shut off from the external atmosphere, into an excited functional position, in which the actuation chamber of the braking force amplifier is subjected to ambient pressure via a flow path, which is freed in this functional position, of the magnetic valve arrangement, but the control passage is shut off from the vacuum chamber. The basic position of the magnetic valve arrangement is associated with the non-actuated condition of the brake system and with its judicious braking operation. The valve arrangement is switched over its excited position when, during actuation of the brake pedal, the threshold value .phi..sub.s of the actuation speed is exceeded. The control passage of the control part is led to the outside via a flexible conduit and a lead-through of the vacuum chamber casing. The magnetic valve is arranged in the outer space of the vacuum chamber and, in its basic position, connects this lead-through to a second lead-through leading back into the vacuum chamber. In the excited functional position of the magnetic valve taken up in the case of triggering by an output signal characteristic of the threshold value .phi..sub.s of the brake pedal actuation speed being exceeded, the first lead-through is shut off from the second lead-through through and is instead connected to the external atmosphere so that the braking force amplifier operates with maximum braking force amplification.
The known brake pressure control device thus far explained is subject, from the functional point of view, to the disadvantage that when the switch-over device responds, the increase in braking force takes place abruptly, and, for a minimum response time, it is practically impossible for the driver to influence the braking force. This can substantially impair a braking operation, at least with respect to driving comfort. A disadvantage from the engineering point of view is that the magnetic valve arranged outside the casing of the braking force amplifier requires substantial installation space which can scarcely be provided in the case of the conventional arrangement of the amplifier/main cylinder unit in the upper, rear part of the engine compartment, where the hydraulic unit of an anti-lock brake system, which is necessary in combination with the brake pressure control device, is also accommodated in many instances.
An object of the present invention, therefore, is to improve a brake pressure control device of the aforementioned type to achieve improved response behavior in the sense of better metering of the braking force and vehicle deceleration with a low space requirement at the same time.
This object has been achieved in a brake pressure control device by providing a rigid force transmission element in mechanical parallel connection to a flexible transmission element in the axial force transmission train, leading from the brake pedal via the reaction piston of the control part, via the reaction element and via the actuation piston of the braking force amplifier to the main cylinder, for the control and actuation forces and for the reaction force resulting from the actuation of the main cylinder, to provide the driver with feedback on the functional condition of the brake system. The force transmission element is associated with a position engaged in the force transmission train as the basic position for judicious braking operation. In that basic position, the rigid force transmission element provides the function of a distance piece between two force transmission train elements following sequentially in the force path direction. A switch-over device is provided for the distance piece to respond when the threshold value (.phi..sub.s) of the pedal actuation speed is exceeded and to then disengage the distance piece from its supporting position, thereby freeing a displacement path within which the reaction piston of the control part can be displaced against a reduced return force in the sense of a pressure build-up control of the control part.
An advantageous feature of the brake pressure control device according to the present invention resides in the fact that, after the switch-over device responds, the braking force remains proportional to the control force with which the driver wishes to select a certain vehicle deceleration. Consequently, the braking force can also continue to be metered. Another advantage is that the functional elements necessary for providing the switch-over device can be realized with little installation volume and can be integrated without difficulty into the control part of the braking force amplifier which, including the brake pressure control device, consequently requires no more installation volume than a conventional brake unit.
If the reaction element is configured as an elastic or quasielastic disc which, shut into an axially limited space by the pressure rod at one end and the reaction piston at the other, behaves approximately as a hydraulic medium, particularly simple and functionally reliable embodiments of the switch-over device of the brake pressure control device can be achieved.
In other embodiments of such switch-over devices utilizing a two-part flange arrangement, the pressure rod providing the transmission of the actuation force to the main cylinder of the brake system is axially supported on the reaction disc of the braking force amplifier. The flange elements of this flange arrangement are axially supported on one another by a distance piece, whose disengagement from the support position makes it possible to switch the amplifier over to a higher value of the amplification factor. A particularly simple embodiment of an actuation device, necessary in this respect, as a control magnet, is also provided.
If the distance piece has an obliquely extending free end surface with the inclined orientation in accordance with one contemplated embodiment and arrangement relative to the flange elements, it is suitable for achieving reliable support of the flange elements on one another. At the same time, it will permit small disengagement strokes of the distance piece because, from a minimum amount of the disengagement stroke of the distance piece from its support position, the disengagement from the same is assisted by the displacement of the flange element carrying the distance piece.
In combination therewith, the present invention provides a particularly advantageous embodiment of the flange part acting on the distance piece with spherical-shaped embodiment, complementary, so to speak, to the inclination of the free end surface of the distance piece, of the flange part assisting the disengagement movement of the distance piece.
Instead of a distance piece which can be disengaged radially out of its support position, which provides a rigid support of the flange parts on one another, a distance piece can be used which cannot be disengaged axially but can be rotated is provided with a profile on its side facing towards the axially displaceable flange part. This profile, rotated into the same orientation as a complementary profile of the movable distance piece, permits the movable distance piece to escape in the axial direction and thereby permits operation of the braking force amplifier with an increased amplification factor. In the basic position of the rotatable distance piece associated with partial braking operation, this rotatable distance piece is "twisted" relative to the movable flange element so that the distance piece and the movable flange part are in contact with one another by the outer, flat envelopes of their profiles. The stroke region becoming free due to twisting of the distance piece is then equal to twice the profile depth of the individual profiles. In a very simple embodiment of such profiles, the rotatable distance piece is, for example, provided with a radial groove and the movable flange part is provided with a radial rib.
Instead of a rotatable distance piece embodied in block form, it is also possible to provide a sleeve-shaped distance piece, which surrounds the axially movable flange part coaxially over a section of its length and has an elongated linkage opening extending obliquely to the direction of movement of the movable flange part. A radial drive trunnion is in engagement with the movable flange part by way of this linkage opening so that an axially fixed support, which can bear loads, can be achieved in the rotational, basic position of the distance piece associated with the partial braking operation because the linkage opening has a short end section extending in the peripheral direction. Consequently, the drive trunnion can be supported on an edge of the sleeve opening extending transverse to the displacement direction of the movable flange part.
Stepper motors, whose rotors are torsionally connected to the respective distance piece and which can be rotated within a limited angular range, are particularly;suitable as rotary drives for the switch-over devices with rotatable distance pieces. Such stepper motors can be realized with advantageously small dimensions in both the axial direction and the radial direction. When such stepper motors are used, it is also possible to electrically control both the disengagement movement and the engagement movement of the respective distance piece out of and into the support position.
Instead of, or in addition to, a two-part flange arrangement of the pressure rod, a two-part reaction piston arrangement can be provided which includes a first reaction piston element, which acts centrally on the reaction disc and on which the pedal push-rod acts, and a second reaction piston element, which is configured as an annular piston and is axially supported on the reaction disc by means of an end surface coaxially surrounding the central surface region where the first reaction piston element acts on the reaction disc, as another embodiment of the switch-over device which permits sensitive metering of the braking force even when the braking force amplifier is operated with its increased amplification factor.
In another preferred embodiment of the switch-over device, the distance piece is configured as a piston-shaped armature of a control magnet provided as the actuation device. It is thus possible to disengage the distance piece against the action of a return spring when current is supplied to the control magnet. In combination therewith, an embodiment of the reaction piston element accommodating the distance piece facilitates the integration of the actuation device in the reaction piston element.